In general, electric fuse elements are used in electric and/or electronic circuits in various configurations for the protection of the circuitry against overcurrent and/or especially against inadmissible heating. When the respective trigger conditions occur in connection with overcurrent or excess temperature, the fuse element is triggered and a particular electric circuit within a system and specifically within the circuit is disconnected or interrupted.
Such fuse elements are generally arranged on a circuit board of the electric or electronic circuit, wherein parts of the fuse element bridge particular points of an electric circuit within the circuitry and in the event of triggering interrupt the electric circuit at such point. For use in electric or electronic circuits fuse elements are manufactured as discrete component parts in large quantities in various configurations and sizes, wherein, depending on the configuration of the fuse elements, the latter can be arranged and mounted manually or by means of an automated operation on a respective circuit board.
Fuse elements that trip with respect to an excess temperature are configured, for example, so that upon reaching a defined excess temperature they open the respective electric circuit, wherein the excess temperature may be reached, on the one hand, by increased electric current and, on the other hand, by heat from outside or by a combination of both possibilities. Fuse elements for thermal triggering include for example in the interior a conductor element which upon excessive current intensity such as an event of short-circuit may entail triggering by designing the conductor element as a fusible conductor. By the great current heat occurring due to the excessive current and/or with excessive head supplied from outside the material of the conductor element fuses at a particular point thus causing the electric circuit to be interrupted. In general, upon reaching the trigger condition and upon triggering of the fuse element, i.e. after fusing of the conductor element, in the case of excessive current and interruption of the electric circuit the component part as such is destroyed.
Such known arrangement is illustrated in FIG. 8 in a simplified and schematic representation. The fuse element is provided in the form of a thermal fuse T including a fusible conductor S which fuses at a comparatively low temperature, is arranged between connecting wires A and is held by the latter. Said array is supported by fastening elements B which in turn are arranged on corresponding conductor paths L and are electrically connected to the conductor paths L. The gap between the conductor paths L shown in FIG. 8 is thus bridged by the thermal fuse T so that a corresponding electric circuit is closed by the thermal fuse T. The entire array is provided on a circuit board P made from insulating material.
In the case of excessive current intensity by the thermal fuse T the fusible conductor S is heated and fuses so that in the area of an interruption point U the electric circuit is interrupted. The electric circuit can be closed again only by replacement of the component part.
Document DE 20 2012 000 571 U1 illustrates a similar fuse element for thermal triggering. In a sealed casing a fusible conductor adapted to a predetermined current intensity which fuses upon reaching the current intensity and interrupts the electric circuit is located on electrically conductive posts or supports. The fused material of the fusible conductor is collected inside the casing. The entire fuse element can be handled with the casing and appropriate terminal contacts as a discrete component part and can be arranged on a circuit board by a mounting operation.
Document DE 196 39 427 A1 illustrates a component part in the form of a resistor in connection with a thermal fuse, with the thermal fuse being in the form of a bridge which bridges two conductor path ends in the area of a gap between the conductor paths.
FIG. 9 illustrates such arrangement of a known fuse element, wherein the conductor path ends L opposing each other via a gap are shown on a circuit board P. Both ends of the bridge-shaped element E are soldered on the respective conductor path end with a low-fusing solder and thus are mechanically and electrically connected to the conductor path ends L and the circuit board P. The bridge-shaped element E takes such shape that in the fastened (soldered) state upon connection of the two conductor path ends L mechanical stress occurs inside the element E according to which mechanical stress at least one side of the bridge-shaped element E can be lifted.
When either of the ends of the bridge-shaped element E soldered with solder fusing at low temperature is released by the solder fusing by heat acting thereon and thus at least one end of the bridge-shaped element E cannot be retained any longer, the respective end of the bridge-shaped element E detaches from the fused soldering point and adopts the position indicated by broken lines in FIG. 9 which corresponds to the trigger position at which the electric circuit is interrupted. In this case, too, the fuse element is destroyed upon occurrence of the trigger conditions and has to be replaced, where necessary.
Document U.S. Pat. No. 5,612,662 illustrates a thermal fuse in combination with a sheet metal strip to which elastic bias is imparted especially by the material selected and the corresponding shape. Both ends of the sheet metal strip are soldered. When the trigger condition is given and fusion of the soldered point is reached due to excessive heating, the element closing the electric circuit is deformed by the elastic bias and the electric circuit is opened. Accordingly, at least one of the two soldering points of the sheet metal strip is fused.
Document DE 196 47 035 A1 discloses a fuse device for protecting an electronic circuit, where equally a sheet metal strip is used as an aid for triggering the fuse device. The sheet metal strip itself does not constitute the fuse element but merely serves as a spring unit being appropriately biased so as to remove further component parts from the circuit board or at least turn them to one side after the respective trigger condition is given. The elastic force of the spring unit is applied to particular component parts, and when the trigger condition such as an excess temperature occurs, a certain number of soldering points of the respective component part is fused so that the retaining force for said component part is significantly reduced and the elastic spring force either removes the component part from the circuit board or at least turns the same to the side. In this way, an electric contact is disconnected. The spring unit is made from said sheet metal strip which is arranged at one side by form fit within an opening in the circuit board.
Finally, the document DE 10 2005 014 601 A1 relates to an electronic subassembly in which a fuse element is provided in combination with spring forces. Apart from given fuse devices in which a metal bracket being biased by a coil spring and bridging conductor paths can be lifted upon fusing one of the soldering points so as to interrupt the electric circuit, further alternatives made from elastic material and taking a specific shape are described. Accordingly, an elastic bracket having plural bends is provided which connects respective conductor paths isolated by a gap and for this purpose is soldered to the conductor paths. On one side, the bracket may engage in an opening of the circuit board by means of a projection. In particular, the contact bracket is fastened and soldered in an original shape on the circuit board and is then brought, by pressure, into a final shape exerting elastic force upon at least one of the soldering points. When the respective soldering point is fused due to excess temperature, then the electric circuit is interrupted by the elastic force lifting the contact bracket and removing the same from the soldering point.
The afore-described known arrangement of a thermal fuse including a specifically shaped contact bracket requires both sides of the contact bracket to be fixed by soldering for fastening the contact bracket so that the latter, on the one hand, can bridge both conductor paths to be connected safely and with certain ampacity and, on the other hand, is fastened at least on one side after triggering. Furthermore, it cannot be ensured whether after plastic deformation in the mounted state the residual elastic deformability and thus the remaining elastic force is safely sufficient to safely disconnect the electric circuit in the case of excess temperature and in the presence of the trigger criterion. In this context it is difficult, on the one hand, to define with certainty a remaining elastic force in connection with the plastic deformation and to realize the same with the component part and, on the other hand, to ensure a constant elastic force of predetermined strength over a quite long service life. Rather, after carrying out the plastic deformation significant tolerances and variations of force have to be expected as regards the remaining elastic force.